a. Field of the Invention
This invention relates to photo-polymerizable compositions which, when exposed to light, form coated films capable of resisting the aqueous solutions employed in etching printed circuit boards, such as solutions of ferric chloride, cupric chloride, ammonium persulfate, nitric acid and so forth, said coated films being strippable only by dipping them in a dilute alkaline aqueous solution for a very short period of time and swelling them thereby.
b. Description of the Prior Art
The compositions which have hitherto been used in etching printed circuit boards were those of the heat drying type containing a volatile solvent consisting essentially of a rosin soluble in a dilute alkaline aqueous solution, a maleic acid-modified rosin and a phenol novolak resin. The dry coated film using therein the composition of this type is readily strippable with a dilute alkaline aqueous solution (which is stripped in a 1-3% NaOH aqueous solution in a short time such as in less than 10 seconds), but, because of its hardness being HB or less in terms of pencil hardness, it suffers many damages during processing steps in fabricating printed circuit boards, thereby inducing a number of breakings of wire in circuits of printed circuit boards. In addition, when screen printing a 0.2 m/m or less narrow linescarrying pattern by using the composition of this type which contains therein a volatile solvent, the ink dries on the screen and causes a clogging of the pores on the screen, whereby the printing of circuits becomes impossible and a number of breakings of wire in circuits of printed circuit boards take place as mentioned above.
U.S. Pat. No. 3,953,214 has already proposed to overcome the aforesaid disadvantages. This patent discloses a composition comprising polyester, hydroxyalkyl acrylate (or methacrylate), a photopolymerizable monomer, a carboxylic acid having 2-36 carbon atoms and a photo-initiator, wherein the polyester acts to impart acid-resistance and hardness to a coated film while the hydroxyalkyl acrylate (or methacrylate) and carboxylic acid having 2-36 carbon atoms act to improve the removability of the film with a dilute alkaline solution. In the composition according to this patent, in order to improve the acid resistance and hardness of the coated film, it is necessary to increase the content of the polyester. However, this is disadvantageous in that the time required for stripping the film with a dilute alkaline solution is prolonged, for instance, such as 10 minutes or more at normal room temperature and 2 minutes or more at 50.degree. C., which is substantially equivalent to ten times or more the stripping time required in the conventional heat drying type compositions. On the other hand, the addition of the hydroxyalkyl acrylate (or methacrylate) and the carboxylic acid having 2-36 carbon atoms in increased quantities for the purpose of shortening the aforesaid stripping time is serviceable for improving the strippability of the cured coated film with a dilute alkaline solution, but acts to deteriorate the acid resisting property of the film and further lowers the hardness thereof, so that said film can not be put to practical use.